Fix RPM builds with a prefix other than /usr.

The location of update-alternatives does not depend on Wireshark's
installation prefix: it's always in /usr/sbin/.

Change-Id: I7dda9cebec83bc64133adfeb5ee6af70dc6dc7ab
Reviewed-on: https://code.wireshark.org/review/3413
Reviewed-by: Jeff Morriss <jeff.morriss.ws@gmail.com>
(cherry picked from commit 644b2e7c69c5289b8a6f524aef84eb7c54cf6058)
Reviewed-on: https://code.wireshark.org/review/3414